Many years ago we dabbled with the use of routing to do square & rectangular ductwork but came up against a few issues and stopped any further development. The main issues were:
1. The vertical axis of the pipe in routing is not actually vertical but at about 45 degrees. After some trial & error we created our square & rectangular ducts with this orientation & the square duct worked OK.
2. The rectangular duct would flip it's orientation when changing direction - from memory I think it worked OK if the bend with up or down, but the profile would flip if the bend went left or right - see attached screen shot.
Any ideas if SolidWorks have pursued this development recently or can offer some tips/work arounds to get the rectangular section to behave correctly (I had a quick play recently and it was still not working correctly).